Steam Assisted Gravity Drainage or “SAGD” (pronounced sag-DEE) is an enhanced oil recovery technology for producing heavy crude oil and bitumen. It is an advanced form of steam stimulation in which a pair of horizontal wells are drilled into the oil reservoir, one a few meters above the other. High-pressure steam is continuously injected into the upper wellbore to heat the oil and reduce its viscosity, causing the heated oil and any condensed water to gravity drain into the lower wellbore, where it can be pumped to the surface.
Generally speaking, high quality, high temperature, and high pressure steam is required. The SAGD process may call for 100% quality, 7,000-11,000 kPA and 238-296° C. temperature steam. Considering oil production volume, and the fact that at least 3 barrels of water are needed for every barrel of oil, the water requirements for SAGD are immense, although water recycling can reduce water consumption considerably.
In addition to requiring large amounts of water, the cost of steam generation is a major contributor to the cost of oil production. The fuel needed to heat water to steam and the transportation of high quality steam to the well pad all contribute to high costs. In addition to the cost of steam generation, each barrel of oil produced in SAGD is co-produced with 3-5 barrels of water, which then must be separated from the oil, and treated before recycling. Water treatment facilities further contribute to cost.
Steam can be produced in various ways, including conventional drum boilers, direct steam generators, and once through steam generators. If the water is sufficiently cleaned, e.g., with evaporator technology, a drum boiler can be used, where the water runs through the boiler more than once, and steam is collected at the top in a drum for distribution, while condensate travels back through the boiler for reheating.
However, the “once-through steam generator” or “OTSG” is more commonly used to provide the steam for SAGD, and other steam based enhanced recovery methods such as cyclic steam generation or “CSS,” because recycled water typically is not clean enough for conventional boiler use.
The OTSG features a single pass of water through the generator coil, where the feedwater is heated and eventually vaporized by a countercurrent of e.g., hot gas produced by the furnace. Regardless of what boiler technology is used, usually the boiler feedwater is preheated by heat exchange with e.g., a hot combustion gas, usually flue gas, or a hot fluid, such as the hot produced fluids. The recapture of otherwise waste heat reduces the overall energy needed to make steam.
The preheated feedwater is then converted to steam in the OTSG by the heat radiated from the furnace, resulting in about 80% quality steam, i.e. the weight ratio of water to steam at the outlet of the generator is about 1:4. The 80% quality steam then goes through a series of liquid-steam separators (also called “flash drums”) to increase the steam quality of OTSG.
Although it is possible to further heat (superheat) the low quality steam to generate 100% steam, this increases fouling of the OTSG as all of the water evaporates and leaves significant solids behind to foul the heat transfer surfaces. Thus, the relatively low steam quality helps to maintain wet conditions in the OTSG tubes in order to reduce fouling and scaling.
The water remaining once the low quality steam and water are separated is called “blow-down” water and has fairly high levels of dissolved organic compounds. Typical blow-down levels are about 20%. Blowdown can be reused in the OTSG, thus saving on overall water usage, but clean boiler feedwater is preferred, because the organics contribute to fouling of the boiler. Thus, blowdown water must be treated before reuse.
There are OTSG designs that include a preheat section, typically called an “ecomomizer”, a vaporizer or “radiant” section, and a “superheater” section so that a high quality steam can be generated within a single OTSG unit. However, these units are quite large, expensive, and as noted, superheating steam leads to fouling unless very clean feed-water is used, which is not usually practical given the imperative to reuse water.
Typically, the OTSG is located at the central processing facility (CPF) and the steam is transported to the well pad for injection because the boiler and preheat equipment is too large to be placed at the well pad. However, the steam-lines that connect the CPF boilers to the pads are costly and limited in length due to pressure loss and steam condensation issues. This contributes further to cost.
Therefore, there is the need for an improved steam generation methods that reduce the cost of steam production, e.g., by eliminating the need for steam-lines, but without overcrowding the well pad.